Self-regulating cooling system

ABSTRACT

A self-regulating cooling system to remove metabolic heat from the coolant loop in a life support system used in space activity. Coolant passes through a sublimator as part of the coolant loop which also includes a pulse pump and a coolant garment. A separate water storage container provides feedwater to the sublimator where it is sublimated along a surface thermally connected to a heat exchange element through which the coolant flows.

United States Patent Inventor Appl. No. Filed Patented Assignee DanielL. Curtis Manhattan Beach, Calif. 825,812

May 19, 1969 Oct. 19, 1971 Litton Systems, Inc. Beverly Hills, Calif.

SELF-REGULATING COOLING SYSTEM 4 Claims, 3 Drawing Figs.

[18. Cl 165/46, 62/315 F281 7/00 244/1 SS;

References Cited UNITED STATES PATENTS 3,152,774 10/1964 Wyatt 244/2.1

3,411,156 11/1968 Feher 2 '2-1 3,463,150 8/1969 Penfold. 244/1 X3,490,718 1/1970 Vary 165/105 3,170,303 2/1965 Rannenherg et al 62/983,212,286 10/1965 Curtis 62/315 3,197,973 8/1965 Rannenberg et al.62/268 3,314,475 4/1967 Valyi 165/170 Primary Examiner-Frederick L.Matteson Assistant Examiner-Theophil W. Streule Attorneys-Alan C. Rose,Walter R. Thiel and Alfred B.

Levine PATENTEUHCT 19 l97| INVENTOR.

DAN/EL L. CURTIS AT TORNEY SELF-REGULATING COOLING SYSTEM BACKGROUND OFTHE INVENTION During manned space exploration a life support system isnecessary for use in either the space vehicle or the space suit. Thelife support system must provide for the controlled dissipation of themetabolic heat of the astronaut as well as any heat absorbed due toexternal radiation such as emanates from the sun. Because of the weightlimitations for a backpack that an astronaut can carry on his space suitand the highlaunching cost per pound, it is extremely important that theweight of the life support system be kept at a minimum, while stillretaining the necessary cooling characteristics.

Previously designed life support systems have generally utilized adynamic form of cooling. Most of these systems use the evaporation of aliquid into space as the principal means of dissipating heat removedfrom the astronauts body by a cooling garment. A coolant flowing throughsmall tubes in the garment carries excess body heat to the coolingdevice where the heat is removed. These systems have generally involvedvery heavy and cumbersome equipment and normally include pumps, valves,and coolant reservoirs requiring complex regulation with many valves andmechanical devices.

State of the art systems have evaporated the liquid, usually water,through a sublimation device wherein the water is allowed to evaporateinto a vacuum thereby absorbing from the coolant system sufficient heatto supply the heat of vaporization to the water. Upon initial exposureto the vacuum, the water freezes and thereafter sublimes while removingadditional heat from the system. Because of the sublimation process, theevaporator is self-regulating, maintaining itself at a temperature ofapproximately 32 F. If the heat-exchanging portion of the sublimator hasgood thermal conductivity from the coolant to the sublimating surfacethen extremely rapid and efficient heat transfer can be achieved.

Conventionally, sublimation-type cooling systems have used a porousmetal as the element in the sublimator which supports the ice layerbeing sublimated and provides the thermal conduction to the ice layer.In such systems, the sublimation occurs in the micropore structure ofthe porous metal, usually a nickel plate. Thus, any impurities existingin the water would automatically be left behind within this microporestructure at the site where vaporization takes place. Over a relativelyshort period of time this slow accumulation of solid residue leads tosystem degradation and performance of the unit is destroyed in arelatively short period of time. To overcome this deterioration inperformance, prior systems required a water so pure as to cast doubt onthe usefulness of sublimator systems during space missions. Anotherproblem plaguing prior systems was the further deterioration ofperformance due to the electrolysis of the metals used. To alleviatethis problem, porous nickel was used as the sublimating surface, becauseof the lower susceptibility of the nickel to degradation throughelectrolysis, even though other metals such as copper have substantiallyhigher heat transfer coefficients. This trade-off resulted in a systemhaving a lower heat transfer coefiicient thereby requiring a largersublimating surface area.

Another basic disadvantage to using porousnickel as the sublimatingsurface was the considerable length of the thermal path from thesublimation surface to the heat exchanger, thereby resulting in arelatively low-efficiency system. To meet the necessary heat removalrequirements it became necessary to construct sublimators which werequite large in size and unduly heavy. These systems were normallycharacterized by a high-pressure drop across the heat exchanger whichneeded large and powerful pumps to maintain more flow through thecoolant loop. The need for waterpurity meant that the feedwater systemhad to be very carefully monitored and where zone temperature controlwas desired, complex regulation systems were required.

It is therefore the object of this invention to provide a cooling devicefor a life support system that is self-regulating and which will beconsiderably lighter and smaller in size than in prior systems.

An additional object of the invention is to provide a cooling systemthat will not be susceptible to performance deterioration over longperiods of operating time and will be impervious to system degradationdue to electrolysis of the various metal elements.

Another object of the invention is to provide a cooling system utilizinga sublimation process where it is possible to separate the sublimationregion from the water distribution and flow control region while stillproviding a short thermal path between the sublimation zone and the heatsource.

Another object of the invention is to provide a cooling system whichwill be characterized by a low-pressure drop of the coolant fluidflowing through the heat exchange portion of the system thereby reducingthe size of the pumps needed for the coolant loop.

Still another object of the invention is to provide a cooling systemwherein the sublimation surface will be characterized by a high-heattransfer coefficient thus resulting in a high-effciency heat removalsystem.

Still a further object of the invention is to provide a cooling systemwhich will be readily adaptable to zone temperature controls wheredifferent temperatures must be maintained over distinct portions of thelife support system.

SUMMARY OF THE INVENTION The aforementioned objects of the invention areaccomplished by channeling feedwater along a felt distributor padsandwiched between an adjustable pressure pad and a metal sublimatingsurface. The sublimating surface is covered by a layer of syntheticfoam. Initially, feedwater spreads in a thin sheet over the surface ofthe metal and any water leaving the metal surface and evaporating intothe foam covering immediately freezes preventing further migration ofthe water into the foam. The net result is a thin layer of ice betweenthe metal and the foam covering which will sublimate away as heat isadded to the system by a heat exchanger which thermally connects thesublimator and the coolant loop. Feedwater flow to the sublimatingsurface is cut off when the felt distributor pads freeze. As heat isadded, the sublimating ice is replaced by the addition of feedwater. Theheat exchange portion of the sublimator is a copper foam or channeledaluminum block through which the coolant liquid or gas flows.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the invention,as well as other objects, aspects, uses and advantages thereof, willclearly appear from the following description and from the accompanyingdrawing, in which:

FIG. I is a perspective view of one embodiment of a cooling system inaccordance with my invention.

FIG. 2 is a portion of a detailed cross-sectional view along lines 22 ofFIG. 1 showing the details of the sublimating apparatus.

FIG. 3 is a perspective view, partially cut away, of a second embodimentof a cooling system in accordance with my invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I a coolingsystem is illustrated which is suitable for use with a life supportsystem such as the one described in my copending application, Ser. No.701,244 filed on Jan. 29, I969 entitled Life Support System" andassigned to the assignee of the instant application. The coolant 10 fromthe thermal control portion of the life support system enters conduit 12in the direction shown and passes through the sublimator 8 to theconduit 14 before returning to the support system. Openings (not shown)are provided in the sides of conduits 12 and 14 to allow the coolant toenter and leave the sublimator 8. Between conduits l2 and 14 the coolant10, which can be either a gas or a liquid, passes through a heatexchanger shown at the cut away portion 23 of the sublimator 8. The heatexchanger can be any metal having a high coefficient of conductivity andthrough which the coolant can easily flow such as a copper foam 22illustrated in FIG. 1. Surrounding the copper foam 22 is a highconductivity solid metal sheet (not shown) which is then covered with anopen cell synthetic foam 19 as the exterior surface of the sublimator.On either side of the sublimator 8, several pressure pads are placed inparallel directions and fastened to sublimator 8 with screws 24. Eachpressure pad 20 has a water feed line 18 to supply feedwater to a feltstrip located along the underside of each pressure pad 20.

The detailed construction of the sublimator may be more clearlyunderstood by referring to FIG. 2 which is partial cross-sectional viewunder one of the pressure pads 20 taken along lines 2-2 of FIG. 1. Thecopper foam 22 is cemented to a copper sheet 30 by a thermallyconductive cement 33 applied along the bottom surface of the sheet 30.Several long strips of felt 21 are placed adjacent the sheet 30 with thefoam 19 disposed to cover the Teflon felt strips 21 and the copper sheet30. The pressure pads 20 are aligned over the felt strip 21 and thescrews 24 adjusted to apply pressure over the foam 19 therebycompressing the foam and the felt strip 21. The pressure pad 20 isconstructed so as to provide a water feed channel 27 through whichfeedwater entering from the water supply line 18 can be supplied to theentire length of the felt 21 lying beneath the pressure pads 20. Thefelt strips 21 act as distributors to evenly spread the feedwater overthe sublimating surfaces 34 and 35 which lie along the upper side of thecopper sheet 30.

The pressure screws 24 are initially adjusted to limit the maximum flowof water through the channel 27 and onto the felt strips 21. Normally,once the pressure screws 24 are initially set they need not be adjustedthereafter. Teflon felt is ideally suited as a flow controller becauseit is one of the few materials in felt form that is anhygroscopic. Whilemost other nonmetallic materials would slowly absorb water thus causingthe material fibers to swell resulting in water blockage, Teflonmaterial will not absorb any water not will it swell.

During the operation of the sublimator 8 shown in FIGS. 1 and 2,feedwater enters each pressure pad 20 through the water feed line 18 andpasses through channel 27 where it is allowed to seep through the foam19 into the felt strip 21. The feedwater initially seeps through theTeflon strip 21 and spreads along the adjacent sublimating surfaces 34and 35 on the copper sheet 30. Because of the vacuum which exists inspace, any water leaving the sublimating surfaces 34 or 35 and passinginto the foam 19 will immediately freeze, forming a porous ice layer onthe underneath side of the foam 19 which prevents further watermigration into the foam layer. The net result is that the water isforced to spread almost uniformly over the sublimating surfaces 34 and35. As the temperature of this surface drops to approximately 32 F. byvirtue of the water vaporization from the foam 19, the water along theentire sublimating surface will freeze and eventually the ice front willextend into the felt strips 21. As soon as the outside of the felt strip21 freezes, water flow from channel 27 onto the sublimating surfaces 34and 35 ceases.

Coolant flowing through the life support system through theaforementioned conduits 12 and 14 will pass through the heat exchanger,in this case, an open cell copper foam 22. The direction of the generalflow of the coolant is represented by arrow 37. It should be noted thatthe actual flow will be a somewhat circuitous path through the foam. Anyheat contained in the coolant will be transferred by the copper foam 22through the thermally conductive cement 33 and the copper sheet 30 tothe ice existing along the sublimating surfaces 34 and 35. Heat reachingthe sublimating surfaces 34 and 35 from the heat exchanger will causesome ice to sublimate away. As the ice lying on the surface of the feltstrips 21 sublimates, feedwater will again flow from the felt strip 21onto the surfaces 34 and 35 until the ice layer is again replenished.

The system is self-regulating because as the heat entering from the heatexchanger unit rises, the sublimation process will speed up thus causinga proportionally greater intake of feedwater from the channel 27.Because there are many sublimation regions in parallel existing alongthe length of the Teflon felt strip 21, the net result is a uniform flowof feedwater into the water feed line 18 and through channel 27 whichwill vary in direct proportion to the heat input to the system.

Another embodiment of a sublimator system is illustrated in FIG. 3. Theelements common to both embodiments are labeled with the same numeraldesignations. As illustrated, the sublimator is constructed similarly tothe above-described embodiment, the exception being that the heatexchange element is an aluminum block 40 having coolant input and outputmanifolds 41 and 47 respectively, connected by a plurality of parallelchannels 46 through which the coolant passes from one manifold to theother. Coolant is supplied to the manifolds from the life supportsystem, as in the previous embodiment, by an input conduit 12 and anoutput conduit 14. The parallel channels 46 are separated by acorresponding number of aluminum fins 43, illustrated in the cutawayportion of the figure. For purposes of simplicity the channels 46 arenot shown except for the portion of the heat exchanger where thesublimating surface is cut away. The aluminum heat exchanger isconstructed by machining the manifolds 41 and 47 and the channels 46into an aluminum plate and then braising an aluminum plate 50 over themachined surface. The result is a solid aluminum block with themanifolds 41 and 47 and the parallel channels 46 contained therein.Constructing the heat exchanger into a unitized block obviates thenecessity for a separate metal plate as a sublimation surface since thesides of the aluminum block are ideally suited for this purpose.

In operation, the embodiment of FIG. 3 will operate substantiallyidentically to the embodiments of FIGS. 1 and 2. However, rather thantaking a labyrinthine course through a metal foam the coolant will flowthrough the parallel channels 46. Tests have shown both embodiments tobe extremely efficient and equally effective.

It should be apparent from the above description of the sublimatoroperation that the major portion of sublimation occurs over the opensublimation surfaces which are adjacent the felt strips and not fromfelt strips themselves. Because only a small fraction of the sublimationprocess actually takes place in the Teflon felt strips, and because thissublimation will take place only along the external surface of thestrips, any residue which is deposited upon sublimation will notinfluence the flow of feedwater through the strips onto the sublimatingsurfaces. Therefore, the purity of the feedwater used in the sublimatorsystem will not be as critical as it was in prior systems. Tests haveshown that feedwater of reasonable purity, such as the commercial gradeArrowhead Puritas Distilled Water (solid residue-1.5 PPM) is sufficientfor efficient operation of the system.

Because the system described above uses copper elements as a heatexchanger rather than a metal having a much lower heat transfercoefficient, it is possible to design a system having an equivalentthermal capacity with substantially smaller and lighter components thanpreviously was available. Also, the sublimation in the instant systemtakes place along a surface rather than in a metal foam material, and amuch shorter thermal path is provided from the heat exchanger to thesublimating surface.

An additional advantage of my sublimator system as exemplified by bothembodiments, is that the sublimator system can be easily adapted toprovide a wide range of temperature control. The thermal capacity of thesublimator can be simply controlled by controlling the feedwater intoeach pressure strip. Where plurality felt distributor strips are locatedon the top and bottom surfaces of the heat exchanger, varying the numberof felt strips in the on" condition (i.e., feedwater flowingtherethrough) will vary the amount of sublimation and thus the heatrejection possible from the heat exchanger. Control can be simplyeffectuated by means of a simple set of onoff valves in the feedwatercirculation system.

Tests run on comparable systems have shown that the present sublimatorsystem constructed in accordance with my invention is capable of a heattransfer coefficient in excess of 300 BTU s hrlftl" F. as compared with160 BTUs hrlft l" F. for a conventional system of comparable design. Inaddition, the present system is substantially lighter and smaller involume and is not susceptible to performance deterioration over longperiods of operating time. The present system is characterized by apressure drop across the heat exchanger which is approximately one-thirdthat of conventional systems. Because of the reduced size and weight ofthe system the lines and control valves needed can be correspondinglysmaller and simpler. This advantage becomes especially important wherethe system is contemplated for use with a remote control unit such asmight be used in a portable life support system to be carried by anastronaut with a control box located on his chest and the sublimatorlocated in a back pack.

With the simplicity of this system, the sublimator is particularlysuited for use with the thermal control system of a space suit lifesupport system and can equally well be utilized in existing and proposedspacecraft thermal control systems. The system is also readily adaptablefor use with either a liquid or gas heat exchanger.

I claim my invention is:

l. A self-regulating cooling system adapted for use in lowpressureenvironments comprising:

a. a heat exchanger for removing heat from a coolant circulatingtherethrough;

b. a metallic sheet having a surface region for sublimating iceintermittently forming thereon from feed water, said metal sheetthermally connected to said heat exchanger for receiving heat therefrom;

c. a layer of open cell foam of low thermal conductivity covering saidsublimating surface for passing water vapor and for supporting a porousice layer formed in regions therein from feed water vaporizing in alow-pressure environment, portions of said ice layer intermittentlyform- 'ing on said sublimating surface and forming in said layer of foamsublimating away through the layer of open cell foam to the low-pressureenvironment in response to heat from said metallic sheet;

d. at least one felt strip positioned between said metallic sheet andsaid layer of open cell foam for spreading feed water over saidsublimating surface region of said metallic sheet and for controllingthe operating temperature of the cooling system about the freezing pointof water, said felt strip having a surface area in contact with saidmetallic sheet which is substantially smaller than the total surfacearea of said metallic sheet, said felt strip spreading feed water overregions of said sublimating surface not covered by a layer of ice, theflow of feed water onto said sublimating surface ceasing whenever iceforms at the water-emitting surfaces of said felt strip and resumingwhenever ice formed at the water-emitting surfaces of said stripsublimates away in response to heat from said metal sheet; and

e. feed water distribution means positioned in juxtaposition with saidfelt strip for supplying feed water to said felt strip; said feed waterdistribution means including means for controlling the maximum possiblerate of flow of feed water from the water-emitting surfaces of saidlayer of open cell foam.

2. Cooling system of claim 1 wherein said heat exchanger includes acopper foam through which a coolant passes.

3. The cooling system of claim 1 wherein said heat exchanger includes;

a. a metal block having input and output manifolds for passing acoolant; and

b. a plurality of parallel disposed channels connecting said input andoutput manifolds for passing a coolant, the coolant in said channelsbeing in thermal contact with said metallic sheet; 4. The cooling systemof claim I wherein said felt strip IS an anhygroscopic material.

1. A self-regulating cooling system adapted for use in lowpressureenvironments comprising: a. a heat exchanger for removing heat from acoolant circulating therethrough; b. a metallic sheet having a surfaceregion for sublimating ice intermittently forming thereon from feedwater, said metal sheet thermally connected to said heat exchanger forreceiving heat therefrom; c. a layer of open cell foam of low thermalconductivity covering said sublimating surface for passing water vaporand for supporting a porous ice layer formed in regions therein fromfeed water vaporizing in a low-pressure environment, portionS of saidice layer intermittently forming on said sublimating surface and formingin said layer of foam sublimating away through the layer of open cellfoam to the low-pressure environment in response to heat from saidmetallic sheet; d. at least one felt strip positioned between saidmetallic sheet and said layer of open cell foam for spreading feed waterover said sublimating surface region of said metallic sheet and forcontrolling the operating temperature of the cooling system about thefreezing point of water, said felt strip having a surface area incontact with said metallic sheet which is substantially smaller than thetotal surface area of said metallic sheet, said felt strip spreadingfeed water over regions of said sublimating surface not covered by alayer of ice, the flow of feed water onto said sublimating surfaceceasing whenever ice forms at the water-emitting surfaces of said feltstrip and resuming whenever ice formed at the wateremitting surfaces ofsaid strip sublimates away in response to heat from said metal sheet;and e. feed water distribution means positioned in juxtaposition withsaid felt strip for supplying feed water to said felt strip; said feedwater distribution means including means for controlling the maximumpossible rate of flow of feed water from the water-emitting surfaces ofsaid layer of open cell foam.
 2. Cooling system of claim 1 wherein saidheat exchanger includes a copper foam through which a coolant passes. 3.The cooling system of claim 1 wherein said heat exchanger includes; a. ametal block having input and output manifolds for passing a coolant; andb. a plurality of parallel disposed channels connecting said input andoutput manifolds for passing a coolant, the coolant in said channelsbeing in thermal contact with said metallic sheet;
 4. The cooling systemof claim 1 wherein said felt strip is an anhygroscopic material.